Modulation of pituitary responsiveness to luteinizing hormone-releasing factor during the estrous cycle of the rat.

This study was performed to investigate pituitary responsiveness to synthetic gonadotropic hormone-releasing hormone (Gn-RH) administered during the 5-day estrous cycle of the rat and to examine whether the observed differences can be attributed to ovarian secretion of estradiol. Gn-RH was injected intra-arterially at 4:00 PM to rats previously anesthetized with sodium pentothal to block the LH surge on proestrus and thus minimize changes in LH secretion which occur throughout the estrous cycle. Pituitary responsiveness was defined as the difference between serum LH concentrations in samples obtained immediately before and 15 min after administration of 200 ng Gn-RH, a time when maximal circulating levels of LH were observed. Administration of Gn-RH was followed by a significant increase in circulating LH on all days of the estrous cycle on which the response was tested (diestrus 2, diestrus 3, proestrus, estrus). Pituitary responsiveness was relatively low on diestrus 2 and estrus and was increased slightly on diestrus 3. On proestrus, however, pituitary response to Gn-RH increased markedly, a phenomenon abolished by ovariectomy at 8:00 AM on diestrus 3. The large increase in pituitary responsiveness observed on proestrus was not restored in such ovariectomized rats when circulating estradiol concentrations were increased and maintained at approximately 150 pg/ml by SC insertion of Silastic capsules containing estradiol-17beta immediately following ovariectomy. Nevertheless, this estradiol treatment consistently elicited an LH surge in another group of ovariectomized rats not treated with sodium pentothal or Gn-RH. Although these observations indicate that an ovarian hormone is essential for the increase in pituitary response to Gn-RH on proestrus, the identity of this hormone remains to be established.